BAMF.
Neville surprised us in the seventh Harry Potter by being such a badass. We now like to refer to him as BAMF. If he lets us.
Shall we review?

"Harry heard a scuffle and a shout, then another bang, a flash of light and a grunt of pain; he opened his eyes an infinitesimal ammount. Someone had broken free of the crowd and charged at Voldemort."
This badass would be Neville.

"The longer Harry looked at Neville, the worse he appeared: one of his eyes was swollen, yellow and purple, there were gouge marks on his face, and his general air of unkemptness suggested that he had been living rough. Nevertheless, his battered visage shone with happiness as he let go of Hermione and said again, 'I knew you'd come! Kept telling Seamus it was a matter of time!'
'Neville what happened to you?'
'What? this?' Neville dismissed his injuries with a shake of the head. 'This is nothing.'"

Finally the most BAMF moment of all:
"In one swift, fluid motion Neville broke free of the Body-Bind Curse upon him; the flaming Hat fell off him and he drew from its depths something silver, with a glittering, rubied handle-
The slash of the silver blade could not be heard over the roar of the oncoming crowd, or the sounds of the clashing giants, or of the stampeding centaurs, and yet it seemed to draw every eye. With a single stroke, Neville sliced off the great snake's head, which spun high into the air, gleaming in the light flooding from the Entrance Hall, and Voldemort's mouth was open in a scream of fury that nobody could hear, and the snakes body thudded to the ground at his feet-"
BAMF!

A forgetful, goofy wizard who became rebellious and bad-ass (see BAMF). A fellow Gryffindor in Harry's year (see Harry Potter), Neville used to be shy, friendless, tearful, forgetful, and a bit of a loser.
However, in Harry Potter and the Deathly Hallows, Neville became one of the most BAMFW (Bad Ass Mother Fucker Wizards) in the world. :)

B.A.M.F. moments of Neville Longbottom, in "Deathly Hallows":

1. He killed Nagini, Voldemort's snake, with the Gryffindor sword, whilst his head was on fire.
2. He stood up to Voldemort, saying boldly he wouldn't join them until 'Hell freezes over'.

3. He stood up to the Carrows, asking them how much Muggle blood they've got.
4. He kept recruiting Dumbledore's Army even after Harry, Ron and Hermione had left.

a kid who seems like a loser who is scared of his granny, but later in the series he becomes more of a bamf. we find out that his parents were bamfs. he hooks up with harry potter's girl before harry potter. he becomes a genius at growing herbs and plants. and he kills voldemort's snake

neville longbottom is the american dream of bamf-erry except that he's british

To get significantly more attractive after going through the process of puberty.

Origin stems from Harry Potter, where the character Neville Longbottom (actor: Matthew Lewis) went from a dumpling shaped (but very sweet) 11 year-old to a daaaamn fine (and still very sweet) 25 year-old today.

The word was popularised by tumblr usage as both a noun and a verb.

Me looking in the mirror: I'd better Neville Longbottom by the time I get to uni, or I'm going to die alone. :/
Person returning home from uni: Is that...? Is that Susan?! Well she certainly Neville Longbottomed.
Person referring to their boyfriend: Nah, I'm not gonna break up with him. Yeah he's kinda pudgy, bit fingers crossed he'll do a Neville Longbottom!